11:15 AM - 11:30 AM
[BCG06-09] Stratigraphy and geochronology of the Birimian Supergroup in the coastal region, Ashanti greenstone belt, Ghana
Keywords:Greenstone belt, Paleoproterozoic, Island arc, West Africa
The Leo-Man Shield of the West African Craton (WAC) is a crustal massif in the sub-Saharan region of West Africa, and several greenstone belts called the Birimian, which were formed about 2.3 to 2 Ga, are distributed in its eastern part of the shield (Milesi et al., 1992). It is not clearly understood that the tectonic development of oceanic island arc crusts (until about 2.2 Ga) that are assumed to be a protoliths of the Birimian greenstone belts (Grenholm et al., 2019). In this study, we focus on the southern Ashanti Belt in Ghana, which only has areas exposing the sedimentary sequences of the Birimian along the coastline in the WAC. The continuous outcrop is expected to provide records of formation process of this Birimian oceanic island arc crust. Especially in the Cape Three Points area, a well-continuous volcanic-volcaniclastic stratigraphy is exposed. The depositional age of them was constrained to before 2172 Ma by zircon age of granitic intrusions (Hirdes et al., 1992). The objective of this study is to clarify the stratigraphy, structure, and age of the volcanic rocks in order to reconstruct the tectonic evolution and stratigraphic history of the area.
Field survey revealed a continuous lithologic succession of about 8000 m in total structural thickness from ultramafic complex in the western part to volcaniclastic metasedimentary rocks in the eastern part. Those rocks strike NE-SW trend with some local variation, and eastward dipping and younging. The eastern flank of the study area is margined by later granitoid. At the western flank of the ultramafic complex is bounded from the western granitoid pluton with a small amphibolite deformation zone. The eastern volcaniclastic sequence is structurally overlying the ultramafic complex. There are well-preserved stratigraphic units that are mainly composed of basalt and andesitic volcaniclastic turbidite deposits and pyroclastic deposits. The lower unit consisting with fine-grained turbidite represents constant input resulted in the rhythmical volcaniclastic turbidity sequence. On the other hand, the upper sequence consisting with coarser and thicker volcaniclastic deposits and lava deposit, which represents larger volcanic event deposits intercalated within the constant sedimentation condition. At the top of the sequence, the volcaniclastic deposits changed to dacitic composition from basaltic-andesitic deposits of the lower and upper parts. These volcaniclastic and volcanic rocks occurred in the area were volcanic-arc basalt type chemical compositions in the discrimination method by Pearce (1973).
The uppermost dacitic tuff was dated and the mean 2172±6.1 Ma (SHRIMP, n=35, MSWD=1.07). Mean age of a porphyry dike intruded into the volcaniclastic beds in the upper middle part of the sequence was 2265±4.6 Ma (SHRIMP, n=48, MSWD=0.95). Leucogabbro occurred in eastern flank of the ultramafic complex was 2282.28±0.57 Ma (CA-ID-TIMS, n=3).
Geology of the study area is estimated to have been formed by long-term continuous volcanic material supply of basaltic, andesitic and dacitic volcanic sediments into sedimentary basins around the island arc volcanoes. That basin is underlain by the ultramafic complex which could be architected at 2.28 Ga as a basin basement. Overlying basaltic and andesitic volcanic sediments were deposited on the basement before 2.26 Ga. They are covered by the dacitic volcaniclastics of 2.17 Ga at the top of the sequence. It likely has a stratigraphic hiatus of about 90 million years between them. The 2.17 Ga dacitic volcanism was synchronous with the surrounding large granitoids (Hirdes et al., 1992), which could be a timing of the magmatic evolution of the region. So, this should be distinguished from the early basaltic and andesitic volcanism in the Ashanti belt, for the better classification of the volcanic stratigraphy.
Geochronological constraints suggest that island-arc igneous activities of the Ashanti belt have started around 2.28-2.26 Ga, which infers the subduction deriving the magmatism in the Ashanti belt was initiated before that. The magmatic episodes in the Ashanti belt can be occurred during the “tectono-magmatic lull (Spencer et al., 2018)”, which may help to figure out mechanism of the rapid continental growth and onset of the supercontinent cycle.
Milési et al. (1992). Precambrian Research, 58(1-4), 305-344.
Grenholm et al. (2019). Earth-science reviews, 192, 138-193.
Hirdes et al. (1992). Precambrian Research, 56(1-2), 89-96.
Pearce and Cann(1973). Earth and planetary science letters, 19(2), 290-300.
Spencer et al. (2018) Nature Geoscience, 11(2), 97-101.
Field survey revealed a continuous lithologic succession of about 8000 m in total structural thickness from ultramafic complex in the western part to volcaniclastic metasedimentary rocks in the eastern part. Those rocks strike NE-SW trend with some local variation, and eastward dipping and younging. The eastern flank of the study area is margined by later granitoid. At the western flank of the ultramafic complex is bounded from the western granitoid pluton with a small amphibolite deformation zone. The eastern volcaniclastic sequence is structurally overlying the ultramafic complex. There are well-preserved stratigraphic units that are mainly composed of basalt and andesitic volcaniclastic turbidite deposits and pyroclastic deposits. The lower unit consisting with fine-grained turbidite represents constant input resulted in the rhythmical volcaniclastic turbidity sequence. On the other hand, the upper sequence consisting with coarser and thicker volcaniclastic deposits and lava deposit, which represents larger volcanic event deposits intercalated within the constant sedimentation condition. At the top of the sequence, the volcaniclastic deposits changed to dacitic composition from basaltic-andesitic deposits of the lower and upper parts. These volcaniclastic and volcanic rocks occurred in the area were volcanic-arc basalt type chemical compositions in the discrimination method by Pearce (1973).
The uppermost dacitic tuff was dated and the mean 2172±6.1 Ma (SHRIMP, n=35, MSWD=1.07). Mean age of a porphyry dike intruded into the volcaniclastic beds in the upper middle part of the sequence was 2265±4.6 Ma (SHRIMP, n=48, MSWD=0.95). Leucogabbro occurred in eastern flank of the ultramafic complex was 2282.28±0.57 Ma (CA-ID-TIMS, n=3).
Geology of the study area is estimated to have been formed by long-term continuous volcanic material supply of basaltic, andesitic and dacitic volcanic sediments into sedimentary basins around the island arc volcanoes. That basin is underlain by the ultramafic complex which could be architected at 2.28 Ga as a basin basement. Overlying basaltic and andesitic volcanic sediments were deposited on the basement before 2.26 Ga. They are covered by the dacitic volcaniclastics of 2.17 Ga at the top of the sequence. It likely has a stratigraphic hiatus of about 90 million years between them. The 2.17 Ga dacitic volcanism was synchronous with the surrounding large granitoids (Hirdes et al., 1992), which could be a timing of the magmatic evolution of the region. So, this should be distinguished from the early basaltic and andesitic volcanism in the Ashanti belt, for the better classification of the volcanic stratigraphy.
Geochronological constraints suggest that island-arc igneous activities of the Ashanti belt have started around 2.28-2.26 Ga, which infers the subduction deriving the magmatism in the Ashanti belt was initiated before that. The magmatic episodes in the Ashanti belt can be occurred during the “tectono-magmatic lull (Spencer et al., 2018)”, which may help to figure out mechanism of the rapid continental growth and onset of the supercontinent cycle.
Milési et al. (1992). Precambrian Research, 58(1-4), 305-344.
Grenholm et al. (2019). Earth-science reviews, 192, 138-193.
Hirdes et al. (1992). Precambrian Research, 56(1-2), 89-96.
Pearce and Cann(1973). Earth and planetary science letters, 19(2), 290-300.
Spencer et al. (2018) Nature Geoscience, 11(2), 97-101.